Process of maintaining brightness in high density wood pulp having a ph6-10 by adding hydrogen peroxide and composition produced thereby



United States Patent PROCESS-F MAINTAINING ERIGHINESS-JIN men DENSITY Ween new HAVING! A pH 6-14) BY: ADDINGHYDROGEN PERQXIDE' AND CBMPOSE'EIQN .PRQDUCED THEREBY.

Fred R. Sheldon, Bufialo, Robert L. .M'cEwen, Williamsville, and Carl E Price," Snyder; NiYi, assignors; by mesne assignments, to Food Machinery and Chemical Corporation, SanJose; Galifi; a corporation of- Delaware No Drawing. Application November 19, 1954 Serial No; 470,128

3. Claims... (Ql. 8-404).

This-invention pertains to-a-method of *makingbrightness-stable pulp and =more-particular-ly= to' a method of making bleached chemical pulps brightness sta'ole during storage.

In pulp and paper mill practice; paper? making pulps are ordinarily'dr-ied or dewatered to-a relatively;high"densityto prepare them forstoring oreconomical shippihg; At this stage, the pulps have generally: been-bleached to the brightness level necessary to make them suitable for preparing a paper furnish. In other words, the pulps at this stage .are or.dinarily at the brightness. lereliioftatzleast 50 to.60 GE units'and'mos't: oftentatithe brightness level: of about 70 to;85 .GEunits; Pulp brightness: normallyis definedin terms of GE units," a; term: whichi-refermo-unim: of reflectance. as, measured: by a General. Electric .refleea. tance meter. according. to. TAPPI. Standard; Test'zlvlethod 'iT2l7m-48. At the .same time. thepulpsswill be: at a relatively: high density, generallyirr the range of :75 to 90%. ment of thenecessary brightness-leveliin: bleaching chem. ieal pulpsis obviously connected with appreciablebleaching costs and it is=ver y important that-a brightness level" once obtained in bleaching the pulp: be preserved sothat a pulpbleached; forexampleeto.a-brightnessof 80 GE- units. will showsubstantially the same brightnessaafter prolonged shipping and storing periods preceding the-timeat which the bleached pulpis actuallyused inimakingmpapaper-furnish. Any loss of-brightnessduring-that pe-- riod is objectionable and'represents partly aavastedeifbrt in bleaching and partly a serious inconvenience to the paper maker.

Overbleaching oh the pulp: toz take: care oflbriglituess loss during; storage: or shipping: is;- alsomobjectionable;

partly because .ofcost but mainly becauseoverbleaching,

is apt to cause fiber damage;

It is, however, well knownsthatxbleachedchemical pulps, dried. or. dewatered j to aphigll'; density: fonistoring 1' and shipping purposes, will; ordinarily sufiert azbrightness decrease. This brightness.--decreaseisi 1 panticularlyrproe.

nouncedin the case'of semi: or fully-bleachedcchemicali pulps in the brightness range-.ofitaboute70ztoi85aGE-units;

Another factor contributing to' .brightnessloss'is. the. heat retained in baled bleached pulp that has been dried on conventional equipments Thebrightness loss in such pulp is still more pronouncedtthan the vbrightnesssloss;v in bleached pulps which are either stored moist: .or partiallydried at low drying temperatures prior cu-storage; Allin.

all, the problem of preservingtheobtained.brightness dur-;

ing storage and shipping of suchipulpdsant' important one:

tothe: pulp and paper-making industry;

We have nowfound that thebleachedibrightness on chemical pulps brought. to an relatiuely high; densityiaften: bleaching can be maintainedvifithere:isi present: inthe; bleached. high density pulp 'a; certain very smallamountv of. hydrogen peroxideaand if; moreover; .theepulp atthisr; stage is within-.- a. critical, pH range: Neither 'thespre'sence'; of hydrogen peroxide alone nor pH -adjusunentalonewill:

Brightness values herein werewdeterminedi 2,822,236 Patented Feb. 4, 1958 r Pulps of higher brightness'an'd higher purity WillprefeF ably be prepared to fall in the lower'part' of 'the'pH range.

indicated; Whereas pulpsoflower brigh'tnessrand"lower purity, forinstance-pulpscontaining quantities ot'non= cellulosic' matter; will preferably fallin the higli'er'part' of thepH range. The particular pfi to be chosen within therange-given will'als'o'be atfected-bythe mode'offtreat ment and theam'ount 'of'hydrogenperoxide-used? In the examples of the disclosure the pH values givenhave been determined by; reslurrying the pulpto' a density of'ab'out" 1 to-3% and measuringthepH-in'th'at-slurry by*electro= metric-means.

Ordinarily hydrogenperoxide'will beused'as the oxidiz= ing agent in this method but other percompoundsyielding hydrogen peroxide when. dissolved in water may also be used. The amount of hydrogen peroxide which is required to maintain-pulp brightnessin the sense ofthis inventionis approximatelybetweenOEOiZ and 025%" Hgflg by Weight;

Ordinarily, an alkali will be'requirediii-addition to the" hydrogen peroxide to adjust the pulp pH to the preferred range-of 6 'to' 10$ The nature of the alkali to'--b'e"used is not critical-'and'alkalies such-as pyropliosphate; tripolyphosphate, other phosphates; borates;sodiiim silicate; sodium carbonate, or'sodium hydroxide may be-used.

If desired, auxiliary agents 'such'assequestering agents; Wetting agents or the-like-maybe used in addition to the" alkali.

Because of the-wide variations in properties-of bleached 1 chemical pulps, preliminary tests will often be necessary to determine the hydrogen peroxide and alkali requirements-suflicient to produce in a-givena bleached higlrdensity pulp the hydrogen peroxide and-pH leveltsuitableffor-i preserving the-initial bleached: brightness. The.- various,- examples given later, illustrate ini detail-x themethodof this invention but, although the H 0 and pH requires ments Will always bewithin the range specified above; optimum. results may. be obtained forrza giyem pulp; only after preliminary testing.

Thevmethod of this invention is generallyapplicablesto. all commercial types of bleachedchemical and bleached. semi-chemical, pulps such 'askraft, soda, or sulfiteipulpsi The particular. manner. in'whichthe. hydrogen peroxide: and, if required, alkali or auxiliary chemical's,. is incor= porated into the-bleached high density-pulp.are-.not -criticaland no particular method of incorporating; these.- agents into the. pulp is claimed here. The hydrogen-per: oxide and the alkali, if required,. may betinconponatedr into the bleached pulp during. or following mechanieali' dewatering or if the pulp is subjected to a drying operation, these chemicals may bezi'ncorporated before, during or. after. drying :the. pulp. The, only; criticalarequirement is thatthe bleachedhighsdensity pulp vcontainthe prea: scribed :amountof. H 9 and. haveathe. proper pH before balingzor otherwise readying thepulp; fonstoragemnd/on shipment;

The. chemicals arequirednbyethez; method of* this: inuenation 'will ordinarily-be use'd in aqueous solutionsv Thesesolutions may be introduced into the pulp by any conventional blending equipment by mechanical pressure rolls or by spraying. The chemicals can also be introduced into the pulp by exposing the pulp to a mist or vapor of those solutions. Wherever the presence of alkali is necessary for pH adjustment; that is to say, wherever more than one chemical is to be introduced into the pulp, the chemicals may be introduced in separate steps instead of being introduced jointly in form of a mixed solution.

The bleached pulp to be treated in accordance with this invention may be in sheet form or in crumb form and the mode of application of chemicals will obviously be dictated by the pulp form; that is, sheet or crumb. However, in either case the actual choice of mode of application of chemicals it not critical and any mode of application suitable or desirable under the particular operating conditions prevailing may be chosen.

As further illustrated by the examples, the process of this invention results in substantially maintaining the bleached brightness of the pulp particularly in the pulp brightness ranges considered here; namely, from about 50 GE units to 80 GE units. The method of invention will not appreciably, if at all, increase pulp brightness beyond the initial bleached brightness of the pulp but provides a simple, efiicient and economical way substantially to preserve and maintain the brightness obtained in initial bleaching of the pulp regardless of the particular bleaching method employed and whether the bleachant was chlorine or peroxide or a combination of chlorine or peroxide bleaching steps.

Example 1 Bleached soda pulp of 81.4 GB units brightness was mechanically dewatered, passed through a pulp drying machine and then baled. After a storage period of 3 weeks, the brightness of this baled pulp had dropped to 78.5 GB units.

Another sample of the same bleached soda pulp of 81.4 brightness was mechanically dewatered. Hydrogen peroxide and sodium tripolyphosphate were then incorporated into the pulp which substantially was dried and at this point contained 0.025% H on an 0. D. basis while the pulp pH was 6.7. After baling, the pulp was stored for three weeks at the end of which time the pulp brightness was 81.7 or substantially the same as the initial brightness.

Example 2 Another sample of the same soda pulp, as used in Example 1, with a bleached brightness of 81.4 was again mechanically dewatered. Hydrogen peroxide and sodium pyrophosphate were then incorporated into the pulp. The pulp was then dried to approximately 75 density and subjected to an accelerated aging test by being exposed for 16 hours in a closed container to a temperature of 60 C. Before aging, the pulp was at a pH 6.5 and contained 0.075% H 0 on a dry pulp basis. After the aging test, the pulp brightness was 81.8, whereas a similar sample of the same bleached soda pulp aged without hydrogen peroxide treatment dropped in brightness upon aging to 77.7.

A sample of the same soda pulp containing, before aging, 0.075% H 0 but having a pH of 5.5 showed brightness drop from an initial brightness of 81.4 to 77.3 during the accelerated aging test. This indicates the importance of proper pulp pH.

Emmple 3 Southern pine kraft pulp was bleached in a conventional manner to 84.1 GE units brightness and dewatered. It was then dried to approximately 75% density, baled and stored for 2 weeks. Due to the drying treatment, initial baled pulp temperature was approximately 60 C. and this temperature dropped to about 25 C. during the storage period. At the end of the storage period, original pulp brightness had dropped from 84.1 to 79.8.

A sample of the same bleached kraft pulp was mechanically dewatered. Hydrogen peroxide, sodium tripolyphosphate, and a sequestering agent (the sodium salt of ethylenediaminetetra-acetic acid) were then incorporated into the pulp. The pulp was then dried to approximately 75% density, baled, and stored for 2 weeks. The initial pulp temperature was again about 60 C. and dropped to about 25 C. during the storage period. The baled puip contained 0.03% H 0 and showed a pH of 7.4. Pulp brightness after storage was 84.2 or essentially the same as the original pulp brightness of 84.1.

Example 4 Kraft pulp bleached in a conventional manner was dried to approximately density, at which time the brightness was 81.4. It was then baled and stored for 4 weeks. At that time the brightness had dropped to 79.2.

Another sample of the same kraft pulp was dried in a dryer to approximately 80% density. The pulp leaving the dryer was sprayed with an aqueous solution of hydrogen peroxide and thereafter contained 0.13% H 0 on an O. D. basis while pulp pH was 8.2. After a 4 week storage period, pulp brightness was still 81.8.

Example 5 Kraft pulp bleached to 83.8 brightness was sheeted and passed through a conventional wet press and then pressed again to about 48% density. The sheet was passed through a dryer, baled, and stored for 26 days. At the end of this storage period, original brightness of 83.8 had dropped to 78.3.

A sample of the same kraft pulp was dewatered by pressing to form a sheet of about 40% density. Into this sheet were then incorporated sodium silicate, sodium hydroxide and a sequestering agent (the sodium salt of ethylenediaminetetra-acetic acid). Then the pulp was passed through a wet press which further increased pulp density to about 48% and at this stage, hydrogen peroxide was incorporated in the pulp sheet which was then passed through a conventional dryer. At this point, the pulp contained 0.07% H 0 and had a pH of 8.7. After baling, the pulp was stored for 26 days at the end of which period pulp brightness was found to be 84.8 or substantially the same as the initial pulp brightness.

Example 6 Sulfite pulp bleached in a conventional manner to a brightness of 85.8 and pressed and dried to approximately 80% density was stored for 8 days. At this time, original pulp brightness of 85.8 had dropped to 79.9.

A sample of the same bleached sulfite pulp was pressed and hydrogen peroxide and sodium tripolyphosphate were incorporated into the pulp which was then dried to approximately 80% density. The pulp before storage contained about 0.1% H 0 and had a pH of about 7.3. After 8 days storage the pulp brightness was found to be 85.8, the same as the initial pulp brightness.

Example 7 Northern hardwood kraft pulp bleached in the conventional manner to about 84 brightness was dewatered and dried to about 75% density, then baled and stored. After storage for 24 days, pulp brightness had dropped from an original 84 to 78.7.

Another sample of the same bleached kraft pulp was dewatered. Hydrogen peroxide, sodium silicate, and sodium hydroxide were incorporated into the pulp before drying to about 75 density. At this point the pulp contained 0.07% H 0 and showed a pH of 9.7. After baling and storing for 24 days, the pulp showed a brightness of 85.6 indicating that original brightness had remained substantially unchanged.

Example 8 A sample of sulfite pulp of 60 brightness was dewatered to about 35% density and then dried at 120 C. to 75% density. Immediately after drying, the pulp was subjected to an accelerated aging test by being exposed in a closed container for 32 hours to a temperature of 50 C. The original brightness of 60 dropped during that aging test to 57.9.

Another sample of the same pulp was dewatered to about 35% density and hydrogen peroxide, sodium silicate, and sodium hydroxide were incorporated before drying the pulp at 120 C. to about 75% density. At this point the pulp contained 0.15% H 0 and had a pH of 8.9. After subjecting this pulp to the same aging test, a brightness of 61.1 was found indicating that original brightness had been maintained.

Example 9 A sample of neutral sulfite semichemical pulp of about 50 brightness was dewatered to 35% density and then dried at 120 C. to result in a density of approximately 75%. This pulp was then subjected to an accelerated aging test by being exposed in a closed container for 16 hours to a temperature of 60 C. The brightness was found to have dropped to 47.

Another sample of the same pulp was also dewatered to 35% density and then hydrogen peroxide and sodium tripolyphosphate were incorporated into the pulp prior to drying at 120 C. to about 75% density. At this time, the pulp contained 0.12% H 0 and showed a pH of 6.7. This pulp was also subjected to the aging test. After aging, the pulp brightness was found to be 50.5 indicating that original brightness had been well maintained.

What is claimed is:

1. The method of maintaining during storage the brightness level in a bleached Wood pulp from the group consisting of bleached chemical and bleached semi-chemical wood pulps, said pulp having been bleached with an amount of a bleach from the group consisting of chlorine bleaches, peroxide bleaches and combinations of chlorine and peroxide bleaches, to provide in said pulp a brightness level of to 85 GE units which comprises dewatering said pulp to a density of to 95%, incorporating in said pulp prior to storage, 0.02% to 0.25% hydrogen peroxide on a dry pulp Weight basis and adjusting the pH in said pulp to a pH in the range 6 to 10.

2. The method of maintaining during storage the brightness level in a bleached wood pulp from the group consisting of bleached chemical and bleached semichemical wood pulps, said pulp having been bleached with an amount of a bleach from the group consisting of chlorine bleaches, peroxide bleaches and combinations of chlorine and peroxide bleaches, to provide in said pulp a brightness level of 50 to GE units Which comprises dewatering said pulp to a density in the neighborhood of 35%, incorporating in said dewatered pulp, from 0.02% to 0.25% hydrogen peroxide on a dry pulp Weight basis and adjusting the pH of said pulp to a pH in the range pH 6 to pH 10 and then, prior to storage, drying said pulp to 75% to density.

3. A new composition of matter, a bleached wood pulp from the group consisting of bleached chemical and bleached semi-chemical wood pulps, said pulp having been bleached with an amount of a bleach from the group consisting of chlorine bleaches, peroxide bleaches and combinations of chlorine and peroxide bleaches to provide in said pulp a GE brightness of 50-85, said pulp having a density of 75% to 95%, a pH in the range pH 6 to pH 10 and having 0.02% to 0.25 hydrogen peroxide on a dry pulp basis whereby the brightness of said pulp is maintained during storage.

References Cited in the file of this patent UNITED STATES PATENTS 

3. A NEW COMPOSITION OF MATTER, A BLEACHED WOOD PULP FROM THE GROUP CONSISTING OF BLEACHED CHEMICAL AND BLEACHED SEMI-CHEMICAL WOOD PULPS, SAID PULP HAVING BEEN BLEACHED WITH AN AMOUNT OF A BLEACH FROM THE GROUP CONSISTING OF CHLORINE BLEACHES, PEROXIDE BLEACHES AND COMBINATIONS OF CHLORINE AND PEROXIDE BLEACHES TO PROVIDE IN SAID PULP A GE BRIGHTNESS OF 50-85, SAID PULP HAVING A DENSITY OF 75% TO 95%, A PH IN THE RANGE PH 6 TO PH 10 AND HAVING 0.02% TO 0.25% HYDROGEN PEROXIDE ON A DRY PULP BASIS WHEREBY THE BRIGHTNESS OF SAID PULP IS MAINTAINED DURING STORAGE. 